Various methods of surgical purification or treatment of blood are used on patients with chronic renal failure. Such methods are used to remove undesirable substances that typically pass through a healthy person's urine. The methods are also used to remove excess body fluid in the patients. Diffusive mass transport is the predominant mechanism in hemodialysis (HD), while convective mass transport through the membrane is involved in hemofiltration (HF). Hemodiafiltration (HDF) is a combination of both methods.
Removing excess body fluid by ultrafiltration is an essential component of dialysis therapy. With the standard form of therapy today, either a fixed ultrafiltration rate (UFR) or a fixed time characteristic of the ultrafiltration rate (UF profile) is selected. The ultrafiltration rate and the ultrafiltration volume during a treatment affect cardiovascular regulation and the fluid compartment of the patient's body. Therefore, ultrafiltration is critical for safety. With uncontrolled withdrawal of fluid at the rate of several liters/hour (L/h), a significant decrease in blood volume may be reached after a few minutes. Such a decrease may result in a severe drop in blood pressure necessitating intensive care measures.
The ultrafiltration rate UFR of a dialyzer increases in proportion to the prevailing transmembrane pressure TMP, where transmembrane pressure is the pressure difference between the average pressure on the blood end and the average pressure on the dialysate end. The ultrafiltration coefficient indicates the amount of fluid removal (ultrafiltration amount) that can be achieved per hour and per mm Hg transmembrane pressure (dimension: mL/h·mm Hg). The ultrafiltration coefficient UFK and the desired fluid removal thus yield the transmembrane pressure TMP to be established.
To increase the patient's safety, existing blood treatment machines typically have a safety device which monitors the transmembrane pressure during the blood treatment to determine whether it exceeds an upper limit or drops below a lower limit, said limits defining a window of safety. An alarm is triggered on leaving this window. Reaching the upper limit may indicate blockage of the dialyzer, and reaching the lower limit may indicate a leak in the fluid system. In a blood treatment with a constant ultrafiltration rate UFR, the window of safety is set manually before starting the blood treatment on the basis of measured transmembrane pressure values. However, if the ultrafiltration rate changes during the blood treatment, the window must be readjusted. Known blood treatment machines allow for a stepwise ultrafiltration profile to be selected so that the window may be automatically readjusted on the basis of new measured values. However, centering the window after a sudden change in ultrafiltration rate generally takes more than one minute with such known devices.
If the ultrafiltration rate is to be regulated continuously, the window must be readjusted constantly. However, if centering the window takes more than one minute, the protective function may be eliminated entirely or at least in part.
German Patent Application No. 34 16 057 A1 describes a hemodialysis machine having a program-controlled device for producing a variable composition of the dialysis fluid. A check is performed during the treatment to determine whether the conductivity of the dialysis fluid is within a predefined window. The conductivity monitoring window is shifted with a change in the composition of the dialysis fluid thereby necessitating readjustment.
German Patent No. 42 39 937 C2 discloses a monitoring a device for hemodialysis equipment intended to prevent ultrafiltration rates that would be harmful to a patient. The disclosed device monitors transmembrane pressure to determine the existence of an undesirable ultrafiltration rate. However, the disclosed device is not suited to reliably monitor transmembrane pressure when the ultrafiltration rate is not constant.